1. Field of the Invention
The present invention relates to an optical element that includes a mirror surface transmitting a part of an incident light beam therethrough and reflecting the other part of the incident light beam, and a projection-type image display device including such optical element.
2. Description of the Related Art
An optical element (e.g. dichroic mirror) including a mirror surface transmitting a part of an incident light beam therethrough and reflecting the other part of the incident light beam has been conventionally known. The dichroic mirror is used as a color separator that separates color component light beams from one another, for example. Specifically, the dichroic mirror is used in a projection-type image display device and separates a light beam emitted from a lamp light source unit into color component light beams of a plurality of colors.
Here, the cutoff wavelength of the dichroic mirror shifts in accordance with the incidence angle of a light beam entering the mirror surface. Specifically, when an incidence angle becomes large, the cutoff wavelength shifts towards a short wavelength side. On the other hand, when the incidence angle becomes small, the cutoff wavelength shifts towards a long wavelength side.
In an optical system to which a dichroic mirror is applied, a target cutoff wavelength is defined. The target cutoff wavelength is a boundary wavelength between a wavelength band of a desired transmission light beam and a wavelength band of a desired reflection light beam. A reference position indicates a position at which a cutoff wavelength reference value indicates the target cutoff wavelength. Here, the cutoff wavelength reference value determines whether a light beam having a reference incidence angle (45°, for example) should be transmitted of reflected.
As described above, since the cutoff wavelength shifts in accordance with the incidence angle, it is necessary to set the cutoff wavelength to be close to the target cutoff wavelength for the entire mirror surface. Accordingly, Japanese Patent Application Publication No. 2001-83636 proposes a technique for setting slants with respect to the cutoff wavelength reference value based on a distance from the reference position, on the mirror surface. Specifically, in a region where the incidence angle is larger than the incidence angle at the reference position, i.e.; in a region where an optical path length of a light beam emitted from a lamp light source unit is longer than the optical path length to the reference position, the cutoff wavelength reference value is set to be a longer wavelength than the target cutoff wavelength. On the other hand, in a region where the incidence angle is smaller than the incidence angle at the reference position, i.e.; in a region where the optical path length of the light beam emitted from the lamp light source unit is shorter than the optical path length to the reference position, the cutoff wavelength reference value is set to be a shorter wavelength than the target cutoff wavelength.
Incidentally, a pair of fly-eye lenses each including micro lenses (cells) is known as an optical element that homogenizes the light quantity distribution of a light beam on a light modulation device (such as a liquid crystal panel), the light beam being emitted from a lamp light source unit. Specifically, the light beam emitted from each of the cells provided in the pair of fly-eye lenses is directed onto the entire surface of the light modulation device.
In an optical system including a lamp light source unit, a pair of fly-eye lenses and a dichroic mirror, the pair of fly-eye lenses is provided between the lamp light source unit and the dichroic mirror. Accordingly, the light beam emitted from each of the cells included in the pair of fly-eye lenses is directed onto the mirror surface of the dichroic mirror.
Here, it is common that the cutoff wavelength reference value is monotonously changed in proportion to the distance from the reference position on the mirror surface in the aforementioned dichroic mirror.
In such dichroic mirror, a color purity of each of the color component light beams separated from one another (a transmission light beam and a reflection light beam) can be increased to some extent, but it is desired to further improve the color purities of the transmission light beam and the reflection light beam.